Upgrading of heavy oil refers to any process of fractionation or treatment of bitumen to increase its value. About one-half of bitumen can be recovered by atmospheric and vacuum distillations, leaving heavy residual hydrocarbons with contaminants. These residual hydrocarbons and other heavy hydrocarbons from various sources can be cracked to give smaller molecules which are more valuable products.
Conventional upgrading technologies include catalytic cracking and thermal cracking or pyrolysis. Catalytic cracking of residual hydrocarbons, particularly asphaltenic hydrocarbons, is difficult as large molecular weight hydrocarbons have low diffusivity into catalyst pores and channels. As well, residual hydrocarbons are rich in coke precursors and catalyst poisons, which severely restrict effective utilization of catalysts. As a result, catalytic methods are less common, and pyrolytic methods are more commonplace in residue upgrading.
Thermal cracking is the oldest and, in a way, the simplest cracking process. It basically aims at the reduction of molecular size by application of heat without any additional sophistication such as catalyst or hydrogen. At temperature levels exceeding about 370° C., the larger hydrocarbon molecules become unstable and tend to break into smaller molecules. By varying the reaction time, temperature and pressure under which a particular feedstock remains under cracking conditions, the desired degree of cracking (conversion) can be controlled.
Coking is a widely-implemented form of thermal cracking, where light products are formed together with significant amounts of coke. Coking, like all pyrolytic methods, is an endothermic process and it is well known that the rate of reaction increases rapidly with increased temperature. However, because coke formation and fouling of heat transfer surfaces also increase rapidly with temperature, viable operating temperatures are conventionally limited to a lower range. Therefore, reaction rates and effectiveness of conventional pyrolytic methods, such as thermal cracking and coking processes are limited.
Additionally, the rate of heating affects coke production. If a feedstock is gradually heated to a pyrolytic temperature, coke formation is significantly higher than if the feedstock is rapidly brought to the same temperature. In U.S. Pat. No. 3,481,720 issued to Bennett on Dec. 2, 1969, a method of pyrolysis of oil shale or oil sands in a concentric reactor-combustor unit is disclosed. Hot “spent” oil shale or oil sands is combined with cold feed oil shale or oil sands to recover heat energy and preheat the feedstock. With oil shale or oil sands, a significant amount of energy is required to heat the shale or sands to reaction temperatures; therefore, heat exchange between the hot “spent” shale or sands with the cold feedstock is essential for energy economy. However, such gradual heating of the feedstock drastically increases coke formation and fouling when processing residual hydrocarbon feedstock, and would be impractical for asphaltenes or residues with high asphaltene contents.
Therefore, there is a need in the art for a process that can achieve fast and selective pyrolysis of heavy hydrocarbons, with high yields of valuable light oils and gases, while significantly reducing coke formation and fouling that constrain conventional pyrolysis.